Examples of a communication system using multiple carriers include an orthogonal frequency division multiplexing (OFDM) system and a DFT spreading OFDM (DFT-S-OFDM) system.
The basic principle of the OFDM system is to divide a data stream having a high transmission rate into a plurality of data streams having a slow transmission rate and simultaneously transmit the data streams using a plurality of carriers. Each of the plurality of carriers is referred to as a subcarrier. Since orthogonality exists between the plurality of carriers of the OFDM system, a receiver can detect frequency components of the carriers even if the respective frequency components are overlapped with each other. The data stream having a high transmission rate is converted into a plurality of data streams having a low transmission rate through a serial to parallel converter, the converted data streams are multiplied by each of the subcarriers, and the respective data streams are added to each other, whereby the resultant data streams are transmitted to the receiver.
The plurality of parallel data streams generated by the serial to parallel converter can be transmitted using a plurality of subcarriers by inverse discrete fourier transform (IDFT). The IDFT can be realized efficiently using inverse fast fourier transform (IFFT).
Since a symbol duration of subcarriers having a low transmission rate increases, temporally relative signal dispersion generated by multi-path delay spread is reduced. Meanwhile, a guard interval longer than delay spread of a channel may be inserted between OFDM symbols to reduce inter-symbol interference. Also, if a part of an OFDM signal is copied in the guard interval and then arranged at a start part of the symbol, the OFDM symbols are cyclically extended to be guarded.
A DFT-S-OFDM system (or single carrier-FDMA (SC-FDMA)) will be described below. The SC-FDMA system is mainly applied to an uplink, and applies spreading using a DFT matrix in a frequency domain before generating an OFDM signal and modulates the spreading result in an OFDM system according to the related art.
FIG. 1 is a schematic view illustrating an example of a DFT-S-OFDM transmitter. As shown in FIG. 1, an input data symbol is converted into a parallel signal by a serial to parallel converter 110 and then input to a DFT spread module 120.
The SC-FDMA system disperses the data symbol ‘s’ using a DFT matrix before transmitting it. This can be expressed by the equation 1.x=FNb×NbS  [Equation 1]
In the equation 1, FNb×Nb is a DFT matrix having a size of Nb, which is used to disperse the data symbol ‘s.’ Subcarrier mapping is performed for a vector ‘x’ by a constant subcarrier allocation mode, wherein the vector ‘x’ is obtained by dispersing the data symbol. The mapped vector is converted into a time domain signal by an IDFT module to obtain a signal to be transmitted to the receiver.
The signal to be transmitted to the receiver can be expressed by the following equation 2.y=FN×N−1x  [Equation 2]
In the equation 1 and the equation 2, N represents the number of subcarriers which transmit an OFDM signal, Nb represents the number of subcarriers for a random user, F represents a DFT matrix, ‘s’ represents a data symbol vector ‘x’ represents a vector which data are dispersed in a frequency domain, and ‘y’ represents an OFDM symbol vector transmitted in a time domain.
In the equation 2, FN×N−1 is a DFT matrix having a size of N, which is used to convert a signal of a frequency domain into a signal of a time domain. A signal ‘y’ generated as above is cyclically prefixed and then transmitted. A method of generating a transmission signal as above and transmitting the generated transmission signal to a receiver is referred to as an SC-FDMA method. The size of the DFT matrix can variously be controlled for a specific purpose.
Hereinafter, an orthogonal frequency division multiple access (OFDMA) system which is an example of a multiple access system will be described below. The OFDMA system realizes multiple access by providing a part of available subcarriers to each user in a modulation system which uses a plurality of orthogonal subcarriers. The OFDMA system provides frequency resources such as subcarriers to each user, wherein each of the frequency resources is independently provided to a plurality of users so as not to cause any overlap.
One of those necessarily required for data transmission in an uplink is pilot transmission. Pilot signals can be classified into two types depending on purpose of use. One of the two types corresponds to channel quality (CQ) pilots for measuring channel quality to perform user equipment (UE) scheduling and adaptive modulation and coding (AMC). The other of the two types corresponds to pilots for channel estimation and data modulation during data transmission. The CQ pilots are transmitted at a previously determined time in a frequency domain. A base station (Node B) identifies channel status of the UE by using the CQ pilots, and performs UE scheduling using the channel status information depending on a given scheduling mode. Accordingly, a plurality of orthogonal channels are required for uplink scheduling of the Node B at a limited time in a frequency domain so that a plurality of UEs within a cell transmit the CQ pilots. As a method of generating orthogonal channels for transmission of CQ pilots, time division multiplexing (TDM), frequency division multiplexing (FDM), code division multiplexing (CDM) or their combined multiplexing can be considered.
Meanwhile, the pilots for channel estimation and data demodulation during data transmission are data pilots transmitted in a specific frequency domain when the UE is scheduled at a specific time in the specific frequency domain and transmits data.
For example, in 3GPP LTE which is one of the standard for mobile communication, sub-frame which is a basic unit of transmission includes one or more pilot transmission blocks for pilot transmission. Since a block to which pilots are transmitted is smaller than or equal to a block to which data are transmitted, a block for pilot transmission will be referred to as a short block (SB). Meanwhile, if one sub-frame has two short blocks, the short blocks will be referred to as SB1 and SB2, respectively.
For scheduling of a frequency domain or high speed scheduling and AMC of sub-frame or its equivalent unit, orthogonal channels are required so that much more UEs transmit pilots. Accordingly, a method of transmitting pilots from the most possible UEs using limited frequency and time resources is required.
For example, in the case that orthogonal channels are simply formed within one sub-frame by time division multiplexing, a peak to average power ratio (PAPR) increases, thereby decreasing an advantage of SC-FDMA in an uplink. Also, even if orthogonal channels are formed by frequency division multiplexing to provide lots of UEs, the number of available UEs is limited due to limited frequency resources.
In the case that orthogonal channels are formed by code division multiplexing, lots of orthogonal codes can be allocated to lots of UEs by use of such lots of orthogonal codes. However, transmission power should be lowered, and if time latency of UE is greater than a certain time period, orthogonality between codes is removed and interference with another UE may be caused.
Meanwhile, since a UE which transmits data should transmit the pilots for channel estimation and data modulation to the base station, a method of properly multiplexing different kinds of pilots from a plurality of UEs and transmitting them is required.